What is the only mandatory hand signal for directing aircraft, regardless of type?

The emergency stop signal is recognized as the only mandatory hand signal for directing aircraft, regardless of the type of aircraft involved. This signal serves a critical function in maintaining safety on the flight line and during ground operations. It is used universally to indicate that an aircraft must halt immediately, ensuring that all personnel involved in ground operations can respond effectively to a potentially hazardous situation.

The importance of the emergency stop signal is underscored by its consistent recognition across different aviation environments, emphasizing safety and clear communication in urgent circumstances. This universality helps prevent accidents and miscommunications that could occur if different signals were used for different aircraft types.

In contrast, the other signals listed, while important for specific maneuvers, do not hold the same level of universality or obligate adherence across all types of aircraft operations. Such signals may be helpful for specific situations but lack the criticality and mandatory nature of the emergency stop.